EXETER, England  Climate scientists from around the world gathered in southwestern England on Tuesday to discuss the crisis of global warming. They are considering three primary questions.

+ For different levels of climate change, what are the key impacts for different regions and sectors and for the world as a whole?

+ What would such levels of climate change imply in terms of greenhouse gas stabilisation concentrations, and what are the emission pathways required to achieve such levels?

+ What technological options are there for achieving stabilisation of greenhouse gases at different concentrations in the atmosphere, taking into account costs and uncertainties?

What is the "Greenhouse Effect?"

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The effect is based on physics models showing that concentrations of certain gases, including carbon dioxide, methane, nitrous oxide, water vapour and chloroflorocarbons (CFCs), help trap the sun's heat in the Earth's atmosphere.

The United Nations Intergovernmental Panel on Climate Change (IPCC) has said that all carbon dioxide emissions, some natural and some caused by mankind's burning of fossil fuels, are increasing and will heighten the greenhouse effect.

Critics say the correlation between atmospheric carbon dioxide and temperature is unproven.

Is Global Warming Real?

A 2001 report by the IPCC said it was very likely that the 1990s were the hottest decade in history and 1998 the warmest year since reliable records began in 1861.

Global average temperatures in the 20th century rose by 0.6 degrees Celsius plus or minus 0.2 degrees, mostly between the years 1910-1945 and 1976-2000.

Some critics of the IPCC report say the temperature changes from the 20th century are within the bounds of normal variability. Others cite faulty research data, and believe surface temperatures alone do not provide the best gauge of climate change.